Cucurbit[7]uril host-guest complexes with cationic bis(4,5-dihydro-1H-imidazol-2-yl) guests in aqueous solution

The host-guest interactions between cucurbit[7]uril and a series of novel cationic bis(4,5-dihydro-1H-imidazol-2-yl)arene and 1-(4,5-dihydro-1H-imidazol-2-yl)- and 1,3-bis(4,5-dihydro-1H-imidazol-2-yl)-adamantane guests have been investigated in aqueous solution using UV-vis and NMR spectroscopy, and electrospray mass spectrometry. With the exception of the 1,3-bis(4,5-dihydro-1H-imidazol-2-yl)adamantane (which binds externally to the CB[7]), these guests form very stable inclusion complexes with slow exchange on the H-1 NMR timescale. The direction and magnitude of the complexation-induced shifts (CIS) in the proton resonances of the guests are indicative of the residence of the hydrophobic core of the guest within the CB[7] cavity and the charged 4,5-dihydro-1H-imidazol-2-yl units outside the cavity adjacent to the carbonyl-lined portals of the host. The CIS values and the inclusion stability constants have been correlated with the nature of the guest core and with the distance between the charges on the terminal 4,5-dihydro-1H-imidazol-2-yl rings.